[mrtg] Re: Not exact values on HTML page

[Alex van den Bogaerdt]

Mike Koch wrote:
> 
> I'm monitoring some variable with an external script as a target. The script
> returns correct values. Values in the log file are the correct ones.
> However, on the web page, the current value written under the graphs is
> slightly different : example :
> 
> In the LOG file :
> 996239888 0 9229
> 996239888 0 9229 0 9229
> 996239587 0 9169 0 9169
> 996239400 0 9105 0 9169
> 996239100 0 9239 0 9343
> 996238800 0 9326 0 9343
> 996238500 0 9316 0 9317
> 996238200 0 9300 0 9316
> ......
> 
> On the WEB page :
> Current Value: 9191

The answer is: resampling/normalizing.

MRTG doesn't work with values as fetched.  It always stores them in
normalized intervals.

> In the LOG file :
> 996239888 0 9229
> 996239888 0 9229 0 9229
> 996239587 0 9169 0 9169
> 996239400 0 9105 0 9169

The normalized intervals are
996239400 to 996239700 (to be stored at 996239700)
996239700 to 996240000 (to be stored at 996240000)

The calculated rate 9169 was valid from 996239400 to 996239587
This defines part of the interval 996239400 to 996239700.
The other part of the interval has a rate of 9229.

So, (996239587-996239400 = ) 187 seconds at rate 9169
(300-187 = ) 113 seconds at rate 9229
Average rate is 187/300*9169 + 113/300*9229 = 9191
(the numbers are truncated to the integer).

If you check your log file now, find the value at time 996239700.
It will be 9191.

cheers,
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